This invention relates in general to firing systems for tubing conveyed perforating guns and in particular to a drop bar firing system including safety means in the event of misfire.
Tubing conveyed perforating is a method of perforating oil well casing with perforating guns suspended from jointed tubing. Under certain completion designs and conditions, tubing conveyed perforating offers significant advantages over conventional wireline perforating.
Completion design plays an important role in the productivity of a well. Depending on reservoir characteristics, wells can be completed naturally, with sand-control measures or with some type of stimulation. The selection of perforating equipment and technique also are important to the productivity of a well. Tubing conveyed perforating provides improved perforator efficiency in all these types of completion. Long intervals can be perforated with large-diameter guns. Underbalanced perforating may be used with well flow initiated upon firing. Reservoir testing is possible with simultaneous perforating and measurement of pressure and flow.
The tubing conveyed perforating system consists of at least one perforating gun run into the well on the bottom of a string of production tubing or drillpipe. The perforating gun or guns may be assembled for any required length and shot density. A packer may be set to isolate the section of the well to be perforated from another section of the well. After gun positioning, firing is performed using one of several types of firing heads. If desired, after firing the gun or guns may be dropped into open casing below the new perforations, allowing access for production logging, use of slickline tools, or stimulation of perforations. Or the guns may be retrieved on the workstring after the well is under control.
One firing technique for tubing conveyed perforating is the drop bar firing system. In this system, a cylindrical weight or sinker bar is dropped or lowered into the tubing and strikes a percussion activated firing head attached to the top of a perforating gun. Vann, U.S. Pat. No. 3,706,344 includes a drop bar embodiment. As shown in FIGS. 1-7 of the Vann patent, a gun firing mechanism 22 is attached between tubing string 13 and casing gun 23. The gun firing mechanism includes a shaft 44 secured by brass shear pins 50 to an upstanding cylinder 42. To fire the system, a weight 45 is dropped or lowered into the tubing string and strikes the top of shaft 44, shearing pins 50 and bringing the lower end of shaft 44 into contact with tapered firing pin 51 within upstanding cylinder 42. Firing pin 51 is driven into rim fire cartridges 54 which activate primer cord detonator 39. The perforating gun shaped charges are activated by the primer cord. An intrinsic problem with the drop bar system is that the drop bar may not contact the firing head with sufficient force to activate the firing means because of debris clogged around the top of the firing head, drop bar sticking within the tubing or for other reasons. This misfiring creates a very dangerous situation as the drop bar must be fished or the tubing string removed from the well with armed charges attached and the drop bar stuck or resting above the armed firing head. Accidental firing at an incorrect depth is extremely damaging to the well and costly. Accidental at firing at the surface is extremely dangerous to well site personnel and equipment.
One object of the present invention is the elimination of this dangerous situation after misfire during drop bar fishing or tubing string removal.